1. Field of the Invention
This invention relates to flat glass bearing a tin oxide coating layer.
2. Description of the Related Art
As is well known, tin oxide coatings often comprise an optional doping agent in order to render the coating electrically conductive, and they may also include minor proportions of other compatible materials for various purposes. The nature and amount of any atoms present other than tin and oxygen should not exceed a limit above which the crystal lattice structure type of the coating differs from that of cassiterite.
Glass sheets bearing a conductive tin oxide coating are widely used inter alia for glazing purposes because of the ability of the tin oxide coating to reduce the emissivity of the coated sheet face in respect of infra-red radiation, especially for wavelengths greater than 3 .mu.m.
It is desirable that a coated sheet used for glazing purposes should be of uniform appearance over its area.
In practice, such coated sheets often exhibit differences in appearance from one region to another. There are various known causes of such variations, including for example variations in coating thickness which may give rise to varying colours over the area of the coating due to interference effects, and variations in coating characteristics giving rise to haze.
Appearance contrasts still occur even when care is taken to make the coating thickness as uniform as possible and to maintain constant conditions during formation of the tin oxide coating.
It has hitherto been considered advantageous, in order to obtain uniform optical properties, that the coating must essentially consist of small crystals. In order to promote the formation of small crystals during pyrolytic deposition of a tin oxide coating layer, it is known to form the tin oxide coating layer from the vapour phase on a thin still-hot preformed coating layer of titanium dioxide which has a similar crystallographic structure to that of tin oxide. The thin titanium dioxide subbing layer tends to be formed of a large number of very small crystals, and thus provides a large number of closely spaced seed points for the growth of a large number of very small tin oxide crystals.